function erase_plot(XD,YD,legend)
plot(XD,YD,new_legend(legend));

function ret=new_legend(legend)
ret=legend;
for j=1:length(ret);
    c=ret(j);
    if strfind('bgrcmyk',c),
        ret(j)='w';return;
    end;
end;
ret=['w',ret];